Many attempts to show indispensable or important functions of the spleen have been unsuccessful until freedivers came along. In fact, a remark made a century ago by a well-known physiologist is still largely true today:
"it (the Spleen) is one of the most obscure and mysterious corners of the human organism"
This organ may be removed from the body without interfering with the vital processes of the body, but this procedure should be avoided at all costs by serious freedivers for reasons outlined in this article.
The spleen lies directly beneath the diaphragm, behind and to the left of the stomach, and is covered by peritoneum. It is purplish in color and varies in size in different individuals, but is usually about 120mm (4.75 inches) long, 70mm wide (2.75 inches) and 25 mm (1 inch) thick. It weighs about 0.2 Kg (less than half a pound).
Its primary function is to destroy red blood cells, a task that it completes together with the liver. It is the organs’ secondary function that is of interest to freedivers however. Because of the huge volumes of blood that circulate through it, the spleen serves as a blood reservoir that plays an integral part of the human diving response, and may help freedivers extend their breath-holds and time at depth.
The splenic reservoir function is observed in many animal species (Weddell Seals, for example, are able to store 24 liters of blood in their spleens) and has been investigated in human breath-hold divers. The spleen acting as a blood reservoir is just one of many physiological adaptations trained freedivers develop that lead to deeper and longer dives. Very simply, their spleen shrinks while diving, causing a release of extra blood cells.
According to William E. Hurford M.D., writing in The Journal of Applied Physiology, the spleens of the Japanese Ama divers (professional female shellfish freedivers) decreased in size by 20 percent when they performed freedives to depths of between 20 and 30m. At the same time their hemoglobin concentration increased by 10 percent. Other studies that investigated human freedivers have corroborated these results. However, in some of these studies, untrained subjects demonstrated a smaller contraction and a less pronounced increase in hematocrit and hemoglobin than the Ama, suggesting that splenic contraction may be subject to a training effect.
The mechanisms that trigger the splenic contraction include peripheral vasoconstriction and a reduction in heart rate, but it is the effect and timeline of the mechanism that holds particular interest for freedivers. Research is a little vague in terms of when splenic contraction occurs. It has been demonstrated in one study that for complete splenic contraction to occur, more than one apnea must be fully initiated. Once this has been performed, the contraction normally occurs within thirty seconds. Upon reaching the surface, the spleen returns to its normal size after about ten minutes. Other studies argue that the spleens contraction and subsequent release of red blood cells is not that immediate and may take up to a quarter hour of sustained freediving. Some researchers state that even this time period is insufficient, and suggest that for freedivers to experience the full effect of splenic contraction they should perform at least thirty minutes of sustained diving.
Regardless of which study is correct, the effect is of particular relevance to freedivers and has implications for those freedivers performing competitively. The mechanism may also explain why dives performed later in a competition/training session are often deeper and longer than those performed at the beginning of a session. It may also be one of the causes of unexplained heart failure in freedivers with a borderline heart condition.
Also of interest to physiologists studying freediving is whether splenic contraction constitutes a part of the mammalian diving reflex and if it is involved in the short-term training effect when performing repeated static apneas. A recent study investigated this by performing a very simple study that involved twenty volunteers. Ten of the subjects had previously been splenectomized (spleens removed for medical reasons) and the other ten had healthy spleens. All performed five maximal duration static apneas with face immersion in cold water (ten degrees Celcius), each apnea separated by a two minute interval. In subjects with spleens, hematocrit and hemoglobin concentration increased during the apneas and returned to baseline within ten minutes of the last apnea being performed. The researchers also observed a delay in the physiological breaking point of apnea in this group. In the splenectomized group neither an increase in hemoglobin or hematocrit was observed, and no delay in the breaking point of apnea was recorded. These results suggest that splenic contraction occurs in humans as part of the diving response and splenic emptying may prolong repeated apneas.
There still exist appreciable margins for improving depth records, and as freedivers continue to dive deeper, and diving physiologists continue to scratch their heads in search of an explanation, an organ that rarely gets a mention might be one of the keys to this successive improvement.